1. Field of the Invention
The present invention relates to a zoom lens and an optical apparatus having the zoom lens, and more particularly to a zoom lens suited for a video camera, digital still camera, a silver-halide film camera, a broadcasting camera or the like, which has a relatively small number of constituent lens elements to make the entirety of a lens system thereof reduced in size while having a high variable magnification ratio, and an optical apparatus having such a zoom lens.
2. Description of Related Art
In recent years, with the advancement of high performance of an image pickup apparatus (camera), such as a video camera or a digital still camera, using a solid-state image sensor, such a CCD or an MOS, a zoom lens having a large aperture ratio including a wide angle of view is desired for the purpose of being used for an optical system of such an image pickup apparatus.
Since, in such an image pickup apparatus, a variety of optical members, including a low-pass filter, a color correction filter, etc., are disposed between the rearmost portion of the zoom lens and the image sensor, a lens system having a relatively long back focal distance is required for the optical system. In addition, in the case of a color camera using an image sensor for color images, a zoom lens excellent in telecentricity on the image side is desired for an optical system of the color camera so as to prevent color shading.
As one method for achieving the above-mentioned requirements, heretofore, there have been proposed a variety of two-unit zoom lenses of the so-called negative lead type each of which is composed of a first lens unit of negative refractive power and a second lens unit of positive refractive power, the separation between the first lens unit and the second lens unit being varied to effect the variation of magnification. In such a zoom optical system of the negative type, the variation of magnification is effected by moving the second lens unit of positive refractive power, and the compensation for the shift of an image plane due to the variation of magnification is effected by moving the first lens unit of negative refractive power. In such a lens construction composed of two lens units, to keep optical performance well, the upper limit of a variable magnification ratio is 2xc3x97 or thereabout.
Further, in order to make the entirety of a lens system in a compact form while having a high variable magnification ratio greater than 2xc3x97, there have been proposed, for example, in Japanese Patent Publication No. Hei 7-3507 (corresponding to U.S. Pat. No. 4,810,072), Japanese Patent Publication No. Hei 6-40170 (corresponding to U.S. Pat. No. 4,647,160), etc., the so-called three-unit zoom lenses in each of which a third lens unit of negative or positive refractive power is disposed on the image side of the two-unit zoom lens so as to correct the various aberrations occurring due to the high variable magnification.
Further, in U.S. Pat. Nos. 4,828,372 and 5,262,897, there is disclosed a three-unit zoom lens composed of three lens units of negative, positive and positive refractive powers, respectively, in which the second lens unit is composed of six lens elements, as a whole, including two cemented lenses.
However, since the above-stated kind of three-unit zoom lens is designed mainly for 35-mm film photographic cameras, it is hard to say that the length of the back focal distance required for an optical system using a solid-state image sensor and the excellent telecentricity are made compatible with each other.
Three-unit zoom lenses of the negative lead type satisfying both the back focal distance and the telecentric characteristic have been proposed in, for example, Japanese Laid-Open Patent Application No. Sho 63-135913 (corresponding to U.S. Pat. No. 4,838,666), Japanese Laid-Open Patent Application No. Hei 7-261083, etc. In addition, in Japanese Laid-Open Patent Application No. Hei 3-288113 (corresponding to U.S. Pat. No. 5,270,863), there is disclosed a three-unit zoom lens in which a first lens unit of negative refractive power is fixed and a second lens unit of positive refractive power and a third lens unit of positive refractive power are moved to effect the variation of magnification.
However, in these zoom lenses, there are such tendencies that the number of constituent lens elements of each lens unit is relatively large, the total length of the lens system is great, and the production cost is high.
Further, in recent years, there has been widely used the so-called barrel-retractable zoom lens in which, in order to make the compactness of a camera and the high magnification of a lens system compatible with each other, the separation between the respective adjacent lens units at the time of nonuse of the camera is reduced up to the separation different from that at the time of use of the camera, thereby lessening the amount of protrusion of the zoom lens from the camera body. However, in a case where, as in the conventional zoom lenses, the number of constituent lens elements of each lens unit is large and, as a result, the length of each lens unit on the optical axis is great, or in a case where the amount of movement of each lens unit during zooming and during focusing is large and the total lens length is, therefore, great, it is sometimes impossible to attain the desired length of the zoom lens as retracted.
Further, in the zoom lens disclosed in Japanese Laid-Open Patent Application No. Hei 7-261083, a convex lens (positive lens) is disposed on the most object side of the first lens unit of negative refractive power, so that an increase of the outer diameter of the zoom lens when made to have a wide angle is inevitable.
In addition, in this zoom lens, since the focusing onto a close object is effected by moving the first lens unit of negative refractive power, there is such a tendency that the construction of a lens mounting mechanism is complicated in combination with the movement for zooming.
Further, in U.S. Pat. No. 4,999,007, there is disclosed a three-unit zoom lens in which each of the first lens unit and the second lens unit is composed of a single lens.
However, in this zoom lens, the total lens length at the wide-angle end is relatively great, and, because the distance between the first lens unit and the stop at the wide-angle end is large, the height of incidence of an off-axial ray of light is large to increase the diameter of a lens element of the first lens unit. Therefore, there is such a tendency that the entirety of a lens system becomes large.
Further, as a problem peculiar to a case where an angle of view at the wide-angle end is enlarged, there is the insufficiency for correcting distortion. In addition, in order to use a zoom lens in association with a high-pixel-density image sensor whose sensitivity is relatively low, the zoom lens is required to have a larger aperture ratio.
On the other hand, as one method of attaining a higher variable magnification ratio while making the length of the back focal distance and the excellent telecentricity compatible with each other, there has also been widely used a zoom lens of the so-called positive lead type in which a first lens unit of positive refractive power is disposed on the most object side.
Among the zoom lenses of the positive lead type, there are disclosed, in Japanese Laid-Open Patent Application No. Sho 62-206516, Japanese Laid-Open Patent Application No. Sho 62-215225 (corresponding to U.S. Pat. No. 4,859,042), Japanese Laid-Open Patent Application No. Sho 62-24213, Japanese Laid-Open Patent Application No. Hei 4-43311 (corresponding to U.S. Pat. No. 5,189,558), Japanese Laid-Open Patent Application No. Hei 5-72472 (corresponding to U.S. Pat. No. 5,572,364), Japanese Laid-Open Patent Application No. Hei 6-34882 (corresponding to U.S. Pat. No. 5,424,869), etc., zoom lenses of the so-called rear focus type, each of which comprises, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, the second lens unit being moved to effect the variation of magnification, and the fourth lens unit being moved to compensate for the shift of an image plane caused by the variation of magnification and to effect focusing.
In general, the zoom lens of the rear focus type has the effective aperture of the first lens unit smaller than that of a zoom lens in which focusing is effected by moving the first lens unit, thereby making it easy to reduce the size of the entire lens system. In addition, the zoom lens of the rear focus type makes close-up photography possible. Further, in the zoom lens of the rear focus type, since a relatively small and light lens unit is moved for focusing, a small driving force is sufficient for driving the focusing lens unit, so that a rapid focusing operation can be attained.
Further, in the general four-unit zoom lens composed of lens units of positive, negative, positive and positive refractive powers, respectively, a high magnification varying action can be performed by moving the second lens unit along the optical axis. For reducing the total length of the zoom lens, it is most effective to decrease the amount of movement of the second lens unit on the optical axis. However, in order to decrease the amount of movement, the refractive power of the second lens unit must be strengthened, so that there is a fear of the deterioration of the image forming performance (optical performance) of the zoom lens due to the strengthening of the refractive power.
Further, in the general four-unit zoom lens composed of lens units of positive, negative, positive and positive refractive powers, respectively, in many cases, the first lens unit, which is the largest one in lens diameter among the four lens units, is composed of three lenses, i.e., one negative lens (concave lens) and two positive lenses (convex lenses), so that the compactness in the radial direction and the optical axis direction of the first lens unit is hindered.
Further, among the zoom lenses of the positive lead type, there are disclosed, in Japanese Laid-Open Patent Application No. Sho 62-247317, Japanese Laid-Open Patent Application No. Hei 10-62687 (corresponding to U.S. Pat. No. 6,016,228), etc., four-unit zoom lenses, in each of which the first lens unit, which would be ordinarily composed of three lenses, i.e., one negative lens and two positive lenses, is composed of one positive lens.
Among the above four-unit zoom lenses, the zoom lens disclosed in Japanese Laid-Open Patent Application No. Sho 62-247317 comprises a first lens unit of positive refractive power arranged to be stationary during the variation of magnification and composed of a single positive lens, a second lens unit of negative refractive power consisting of one cemented lens composed of one positive lens of meniscus form and one negative lens of bi-concave form cemented together and arranged to move monotonically toward the image side during the variation of magnification from the wide-angle end to the telephoto end, a third lens unit of positive refractive power consisting of one cemented lens and one positive lens and arranged to move monotonically toward the object side during the variation of magnification from the wide-angle end to the telephoto end, and a fourth lens unit of positive refractive power arranged to be stationary during the variation of magnification.
However, in the zoom lens disclosed in Japanese Laid-Open Patent Application No. Sho 62-247317, since the second lens unit is arranged to move monotonically toward the image side during the variation of magnification from the wide-angle end to the telephoto end, the above-mentioned inconveniences are still not dissolved.
Further, in the zoom lens disclosed in Japanese Laid-Open Patent Application No. Hei 10-62687, the second, third and fourth lens units are moved during the variation of magnification to make the second, third and fourth lens units share a magnification varying action with each other, so that the amount of movement of each of the second, third and fourth lens units can be made small without deteriorating the image forming performance. However, since the second lens unit still has a main portion of the magnification varying action, the above-mentioned arrangement is insufficient for reducing the total length of the zoom lens.
In view of the above-mentioned drawbacks of the conventional zoom lenses, an object of the invention is to provide a zoom lens which is suited for a photographic system using a solid-state image sensor, has a high variable magnification ratio despite being compact and small in diameter with less constituent lens elements, and has excellent optical performance, and to provide an optical apparatus having the zoom lens.
To attain the above object, in accordance with an aspect of the invention, there is provided a zoom lens comprising a lens unit A of negative refractive power, and a lens unit B of positive refractive power disposed on an image side of the lens unit A, the lens unit B comprising two cemented lens components and consisting of not more than five lens elements, wherein the separation between the lens unit A and the lens unit B varies during zooming.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.